Near field optical microscopic examination of a biological specimen

ABSTRACT

An optical system useful, e.g., for near-field scanning optical microscopy is provided. The system incorporates a probe having improved properties. In one embodiment, the probe comprises a tapered and partially metallized portion of a single-mode optical fiber.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 07/925,809 filed Aug. 4,1992, which is a continuation-in-part of U.S. application Ser. No.07/771,413, filed Oct. 10, 1991, now abandoned, continuation-in-part ofU.S. application Ser. No. 07/615,537 filed Nov. 11, 1990, abandoned.

FIELD OF THE INVENTION

This invention relates to optical microscopes in which a small aperture,typically smaller than an optical wavelength, is positioned within theoptical near field of a specimen, i.e., within a distance of less thanabout an optical wavelength of the specimen, and the aperture is scannedin raster fashion over the surface of the specimen to produce atime-varying optical signal that is detected and reconstructed toproduce an image having extremely high resolution. The invention furtherrelates to methods of use of such microscopes for inspecting workpiecesin the course of manufacturing processes.

ART BACKGROUND

Microscopes employing conventional optical imaging systems cannotresolve features substantially smaller than about one-half an opticalwavelength. That is, when both the entrance pupil of the microscopeobjective and its distance from the specimen are substantially largerthan a wavelength, diffraction effects limit the smallest resolvableseparation between a pair of object points to 0.5λ/N.A., where λ is theoptical wavelength and N.A. denotes the numerical aperture of theobjective, which, as a practical matter, is limited to values of about1.6 or less. (F. A. Jenkins and H. E. White, Fundamentals of Optics,Third Edition, McGraw-Hill Book Co., New York, 1957, pp. 306-308.)

Electron microscopy has been successful in resolving features muchsmaller than those resolvable with conventional optical microscopy.However, for some applications, electron microscopy suffers from certaindisadvantages. For example, the specimen must be enclosed in anevacuated chamber. Where the electron microscope is employed as adiagnostic device on a production line for, e.g., semiconductor wafers,the time required to vent and evacuate the chamber may detractsignificantly from the manufacturing throughput. As another example,certain features of a specimen that are detectable by optical microscopymay nevertheless be invisible to an electron microscope, becausedifferent contrast mechanisms are involved. As yet another example, thevacuum environment, or exposure to the electron beam, may be destructiveto the specimen.

A number of researchers have investigated the use of optical scanning tocircumvent the inherent limitations of conventional optical imagingsystems. That is in so-called near-field scanning optical microscopy(NSOM), an aperture having a diameter that is smaller than an opticalwavelength is positioned in close proximity to the surface of aspecimen, and scanned over the surface. In one scheme, the specimen isreflectively or transmissively illuminated by an external source. Aportion of the reflected or transmitted light is collected by theaperture and relayed to a photodetector by, for example, an opticalfiber. In an alternate scheme, light is relayed by an optical fiber tothe aperture, which itself functions as miniscule light source forreflective or transmissive illumination of the specimen. In that case,conventional means are used to collect and detect the selected ortransmitted light. In either case, the detected optical signal isreconstructed to provide image information.

Thus, for example, U.S. Pat. No. 4,604,520, issued to W. D. Pohl on Aug.5, 1986, describes an NSOM system using a probe made from a pyramidal,optically transparent crystal. An opaque metal coating is applied to thecrystal. At the apex of the crystal, both the tip of the crystal and themetal coating overlying the tip are removed to create the aperture,which is essentially square and has a side length less than 100 nm.

Also described in U.S. Pat. No. 4,604,520 is an alternative aperturemade from a single-mode optical fiber. One planar end of the fiber ismetallized, and a coaxial hole is formed in the coating so as to justexpose the core of the fiber.

In a somewhat different approach, U.S. Pat. No. 4,917,462, issued to A.L. Lewis, et al. on Apr. 17, 1990, describes a probe formed from apipette, i.e., a glass tube that is drawn down to a fine tip and coatedwith an opaque metal layer. After drawing, the pipette retains a hollowbore, which emerges through both the glass and the overlying metal layerat the tip. The resulting metal annulus defines the aperture. Theaperture may be smaller than the bore defined in the glass, as a resultof radially inward growth of the metal layer.

In yet another approach, R. C. Reddick, et al., "New form of scanningoptical microscopy," Phys. Rev. B, 39 (1989) pp. 767-770, discussed theuse of a single-mode optical fiber as a probe for so-called photonscanning tunneling microscopy (PSTM). The fiber tip is sharpened byetching, and the tip is optionally coated with an opaque material todefine an aperture on the very end of the fiber tip. (It should be notedthat PSTM differs from transmission or reflection microscopy in that theillumination system is adapted to produce total internal reflection ofthe PSTM specimen. The probe tip is brought into the evanescent opticalfield above the sample. A portion of the optical energy in theevanescent near field is coupled into the probe and propagates throughit, ultimately reaching a detector.)

One drawback of most of the above-described methods is that light istransmitted through the probe with relatively low efficiency. As aconsequence, signal levels are relatively low. In some cases, aperturesmust be made larger in order to compensate for low signal levels. Thismeasure is undesirable because it results in lower resolution. (PSTMgenerally offers relatively high signal levels, but resolution isgenerally no better than can be achieved by conventional opticalmicroscopy.)

For example, when light is transmitted from a source to the aperturethrough an uncoated pipette, the optical field has a substantialamplitude at the outer walls of the pipette. In order to confineradiation, it is necessary to coat the walls with metal. However,attenuation occurs as a result of absorption in the metal coating.Moreover, metal coatings are prone to imperfections, such as pinholes,that permit optical leakage. When this tendency is countered byincreasing the metal thickness, the length (i.e., the thickness in theaxial direction of the pipette) and outer diameter of the metal annulusdefining the aperture are also increased. As a result, optical lossesdue to absorption and evanescence in the metal annulus are increased andthe size of the tip is increased. Enlarging the tip makes it moredifficult to scan narrow concave topographical features of the specimenwhile maintaining close proximity to the specimen surface.(Significantly, the problem of excessive tip size due to metaldeposition also applies to constant-diameter optical fiber probes of thetype having an aperture defined by a hole in a metal layer coating theend of the fiber.)

Analogous problems occur when light is transmitted in the oppositedirection, i.e., from the specimen (by transmission or reflection) intothe aperture, and thence through the pipette toward a detector. Theoptical signal is attenuated in the aperture region, as described. Aportion of the optical signal may be lost by absorption in the metalcoating of the walls, and through pinhole leaks in the metal coating,also as described. Moreover, scattered light may enter the pipettethrough pinhole leaks, resulting in an increased noise level at thedetector.

Still further problems occur because a portion of the light that passesthrough the pipette toward the aperture is reflected from the outerglass walls of the pipette. After suffering multiple reflections, someof the light may undergo a reversal of propagation direction. As aconsequence, the amount of light incident on the specimen may bereduced.

Probes made from pyramidal crystals suffer difficulties that areanalogous to those described above in connection with pipette-typeprobes.

Problems that occur when the probe is a single-mode fiber having asharpened (e.g., by etching) conical tip and no metal coating aredescribed, e.g., in C. Girard and M. Spajer, "Model for reflection nearfield optical microscopy," Applied Optics, 29 (1990) pp. 3726-3733. Oneproblem is that a portion of the light passing through the fiber towardthe tip may be reflected by, and then transmitted through, the sides ofthe conical taper. A second problem is that the sides of the taper maycapture undesired optical signals that can propagate through the fiber,resulting in an increased noise level at the detector.

In view of the foregoing discussion, it is apparent that investigatorshave hitherto been unsuccessful in providing an NSOM probe that combinesefficient transmission of light (i.e., transmission that is relativelyfree of attenuation due to optical interactions with the walls of theprobe) with relatively small tip dimensions, high resolution, and highreliability.

SUMMARY OF THE INVENTION

In one aspect, the invention involves an optical system, comprising: aprobe, at least a portion of which is optically transmissive at least ata given wavelength, the probe having a distal end; an optical aperturedefined in the distal end, the aperture having a diameter smaller thanthe given wavelength; means for optically coupling a light source to theprobe such that at least some light emitted by the source, at least atthe given wavelength, enters or exits the probe through the aperture;and means for displacing the probe relative to an object, characterizedin that: (a) the probe comprises a portion of a single-mode opticalfiber having a core and a cladding, the cladding having an outersurface, there being associated with the fiber a guided dielectric mode;(b) the fiber has a taper region that is adiabatically tapered, at leasta portion of the taper region being capable of guiding light of at leastthe given wavelength; (c) the taper region terminates in a substantiallyflat end face oriented in a plane substantially perpendicular to thefiber, the aperture being defined in the end face; (d) the claddingouter surface in the taper regions substantially smooth; (e) at least aportion of the cladding outer surface in the taper region is coated withmetal, defining a metallic waveguide portion capable of guiding ametallic mode, there further being defined a cutoff diameter for themetallic mode; and (f) the end face diameter is less than or equal tothe cutoff diameter.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic drawing of an exemplary optical system useful fornear-field scanning optical microscopy.

FIG. 2 is a schematic drawing of an alternative, exemplary opticalsystem useful for near-field scanning optical microscopy,

FIG. 3 is a schematic drawing of a prior-art optical fiber probe.

FIG. 4 is a schematic drawing of a prior-art optical fiber probe.

FIG. 5 is a schematic drawing of an optical fiber probe according to oneinventive embodiment.

FIG. 6 is depicts an exemplary method for metallizing an optical fiberprobe.

FIG. 7 is a schematic depiction of an optical aperture according to oneinventive embodiment.

FIG. 8 is a schematic depiction of an alternative optical aperture.

FIG. 9 is a flowchart illustrating a manufacturing process according toone inventive embodiment.

FIGS. 10 and 11 schematically depict alternate embodiments of theinventive probe in a planar waveguide instead of an optical fiber.

DETAILED DESCRIPTION

In one embodiment, the invention involves an optical system. Turning toFIG. 1, such an optical system includes a light source 10, a probe 20,displacement means 30 for displacing the probe relative to an object 40disposed, exemplarily on a stage 50, adjacent the probe tip 60. Theoptical system further comprises means for optically coupling lightsource 10 to probe 20. In the example illustrated in FIG. 1, the opticalcoupling is provided by a single-mode optical fiber 70 extending betweenlight source 10 and probe 20. (Fiber 70 may, in fact be integral withprobe 20.) Light source 10 is exemplarily a laser. Light from source 10is readily injected into the optical fiber by way, e.g., of asingle-mode coupler 80, which includes a microscope objective 90 and afiber positioner 100. A mode stripper 110 is also optionally included inorder to insure that only the single mode in the core is propagated tothe probe, and not other modes in the cladding. The displacement means30 may, for example, be a piezoelectric tube adapted for moving theprobe vertically as well as in two orthogonal lateral dimensions.Alternatively, the displacement means may be mechanical or piezoelectricmeans for moving the stage rather than the probe, or some combination ofstage-displacement and probe-displacement means.

One possible use for an optical system as described is for directwriting. That is, the sample surface proximate the probe tip may becoated with a photosensitive layer that is capable of being exposed bylight emitted from the light source. An exposure pattern is created inthe photosensitive layer by displacing the probe relative to the sample,while light from the light source is continuously or intermittentlyemitted from the probe tip.

A second possible use for an optical system as described is for imagingof the sample in a so-called "illumination" mode. According to thisapplication, light from the probe tip is transmitted through the sampleand collected below the stage (as shown in FIG. 1) by microscopeobjective 120. (Illustrated is an illumination-transmission mode; anillumination-reflection mode is also readily practiced.) The collectedlight is directed into detector 130, which is exemplarily aphotomultiplier tube. For visual positioning of the probe, it is alsodesirable to include a beamsplitter 140, which directs a portion of thecollected light into an eyepiece 150. Significantly, when the sample isscanned by a raster-like displacement of the probe, the signals fromdetector 130 can be reconstructed to produce an image of the sampleportion that has been scanned. Such scanning methods are employed innear-field scanning optical microscopy (NSOM), in which the probe tip isbrought to within a very small distance of the sample surface, typicallyless than a wavelength of the light emitted by the light source. NSOMprovides very high optical resolution by also employing an aperture inthe probe tip that is very small, also typically less than onewavelength. NSOM apparatus is well-known in the art, and is described,for example, in U.S. Pat. No. 4,604,520, issued to W. D. Pohl on Aug. 5,1986, and in U.S. Pat. No. 4,917,462, issued to A. Lewis, et al. on Apr.17, 1990.

Yet a third possible use for an optical system as described is shown inFIG. 2. In the arrangement of FIG. 2, the probe tip serves as acollector of light rather than as an emitter of light. Such anarrangement is useful, e.g., for NSOM imaging in a so-called"collection" mode. (Illustrated is a collection-reflection mode. Acollection-transmission mode is also readily practiced.) Light fromlight source 10 is directed via tilted mirror 160 and tilted annularmirror 170 to annular objective lens 180. Lens 180 focuses the lightonto the sample surface. Light reflected or emitted from the surface iscollected by the probe tip and directed via fiber 70 and objective 120to detector 130. Reflection mode NSOM is described, for example, in U.S.Pat. No. 4,917,462, cited above.

The detector (or, more generally, the transducer) 130 converts thedetected light to electrical signals. These signals are readily used tocreate a two-dimensional image on a video display device such as acathode-ray tube. For such purpose, a scan generator is used to controlthe displacement of the probe relative to the object and to provide areference signal for constructing the displayed image. The electricalsignals generated by transducer 130 are typically analog signals. Theseare optionally converted to digital signals before they are displayed.In such a case, a digital memory is optionally provided for storing thedigitized signals, and a digital processor is optionally provided toprocess the digitized signals (for, e.g., image enhancement) before theyare displayed.

One possible form of probe 20 is a single-mode optical fiber. Opticalfiber probes have, in fact, been disclosed in the prior art. FIGS. 3 and4 show examples of such fiber probes. FIG. 3 shows an untapered,single-mode optical fiber 190, having an annular cap of opaque material,such as metal, deposited so as to cover the tip. The orifice 210 at thecenter of the annulus defines the optical aperture of the probe. FIG. 4shows a bare optical fiber 220 that is tapered, for example by chemicaletching.

We have discovered that an improved probe 230 is readily made by heatinga single-mode optical fiber to soften it, and drawing the softened fiberto form a tapered fiber. After drawing, at least a portion of thetapered fiber must be coated with an opaque material 280, exemplarilymetal. With reference to FIG. 5, the tip 240 of such a drawn fiber istapered at an angle β, and terminates in an end flat 250.

The optical fiber comprises a cladding 260 and a core 270. Althoughspecific cladding and core compositions are not essential to theinvention, an exemplary cladding composition is fused silica, and anexemplary core composition is doped, fused silica having a higherrefractive index than the cladding. Although specific cladding and coredimensions are also not essential to the invention, an exemplary corediameter is about 3 μm, and an exemplary cladding outer diameter isabout 125 μm. One guided mode, the fundamental or HE₁₁ mode, isassociated with a corresponding single-mode fiber that has not beentapered. Such a mode is characteristic of a cylindrical dielectricwaveguide, and for that reason is hereafter referred to as a dielectricmode.

When an optical fiber is tapered by heating and drawing, both the corediameter and the cladding outer diameter are decreased. The fractionalchange in the core diameter is approximately equal to the fractionalchange in the cladding outer diameter. (In other words, the crosssection of the fiber changes in scale only.) Significantly, the angle atwhich the core is tapered is substantially smaller than β. For example,in a linearly tapered fiber having the exemplary dimensions, the tangentof the core taper angle is only 3/125, or 2.4%, times the tangent oftaper angle β. For this reason, even for relatively large values of β,for example values of 30° or even more, the core will have an adiabatictaper, as discussed below.

In the untapered fiber, the electric field of the dielectric mode islargely confined to the core, and it falls to a very small amplitude,typically less than 10⁻¹⁰ times the peak amplitude, near the claddingouter surface. That is not necessarily the case in a tapered fiber. As aguided light wave propagates into the taper region, it encounters aprogressively narrowing core. Eventually, the core becomes too small tosubstantially confine the guided mode. Instead, the light wave is guidedby the interface between the cladding and the surrounding material,which may be air, or a metal such as aluminum. As discussed above, thecore will generally be tapered at a small enough angle for the mode to badiabatic. By adiabatic is meant that substantially all of the energy ofthe initial HE₁₁ mode remains concentrated in a single mode, and is notcoupled into other modes, particularly radiation modes, which arecapable of causing optical losses by radiation.

Initially, the guided mode that escapes from the core substantiallyretains the properties of a HE₁₁ mode. In particular, the amplitude ofthe guided electromagnetic field is relatively small at the claddingouter surface. However, as the fiber diameter continues to decrease, thefield amplitude at the cladding outer surface increases relative to thepeak amplitude within the fiber. Eventually, the field attains arelatively large amplitude at the cladding outer surface. Such a modecould be guided, for example, by the interface between the cladding andsurrounding air. However, such an arrangement is undesirable.

Because the electric field extends significantly outside of thecladding, a significant amount of optical leakage can be expected. Thisis undesirable because it reduces the efficiency with which light ischanneled both to and from the probe tip. Moreover, light leaking fromthe cladding walls can ultimately fall upon portions of the sample lyingrelatively far from the probe tip, resulting in unintended exposure ofthe sample, or in increased background levels at the detector.Furthermore, if the cladding out walls are surrounded only by air, straylight can enter the fiber, again resulting in increased backgroundlevels at the detector (if, for example, the probe is being used tocollect light from the sample).

For the above reasons, it is desirable to coat at least the terminalportion of the taper, from which optical leakage is a significantfactor, with an opaque material, exemplarily a metal such as aluminum.Significantly, the guided mode in such a metallized portion hascharacteristics typical of a guided mode in a metal, rather than adielectric, waveguide. Thus, for example, an initial HE₁₁ dielectricmode may be converted to a TE₁₁ metallic mode as it approaches andenters the metallized region. Significantly, in a waveguide having anadiabatically tapered core, the HE₁₁ mode can couple with relativelyhigh efficiency (typically, greater than 10% efficiency) to the TE₁₁mode. That portion of the fiber in which, if the fiber were bare, theguided mode would have a significant amplitude at the fiber outersurface, but would still retain substantial HE₁₁ character is herereferred to as the "transition region." In view of the foregoingdiscussion, it is clearly desirable (although not absolutely required)to metal-coat the fiber throughout the transition region, and from thetransition region to the distal end of the fiber. It should be noted inthis regard that where a significant amount of optical energy is presentin radiation modes, relatively thick metal coatings are required inorder to substantially eliminate the possibility of pinhole leaks and,in view of the finite conductivity of any actual coating, thepossibility of penetration of the metal by the electromagnetic field.This is undesirable both because the coating thickness may hinderextremely close approach of the probe tip to the sample, and becausevery thick metal coatings may develop granularity that tends to promote,rather than inhibit, the occurrence of pinhole leaks. By contrast, it isgenerally expected here that relatively little energy will couple intoradiation modes, and therefore relatively thin metal coatings, typically750-1500 Å, and preferably less than about 1250 Å will generallysuffice.

Generally, the guided metallic mode will initially be a propagatingmode. However, as the fiber diameter continues to decrease, the guidedmode may eventually be transformed to en evanescent mode exhibitingrelatively strong attenuation in the propagation direction. Such atransition may be associated with a characteristic quantity herereferred to as the "cutoff diameter." The cutoff diameter is thecladding outer diameter of a given tapered waveguide at which thetransition from a propagating to an evanescent mode would occur if thewaveguide were coated with an infinitely conductive metal. Generally,for the TE₁₁ mode, the cutoff diameter is roughly equal to one-half theguided wavelength. That portion of the fiber extending from the cutoffdiameter to the probe tip is here referred to as the "evanescentregion."

The cutoff diameter of a TE₁₁ mode in, e.g., a circularly cylindricalwaveguide is readily predicted from the theory of metallic waveguides asdiscussed, for example, in J. D. Jackson, Classical Electrodynamics, 2dEdition, John Wiley and Sons, Inc., New York, 1975, p. 356. It is equalto the quantity χ/k₀ n, where k₀ is the free-space wavenumber of theguided light, n is the refractive index of the waveguide (with referenceto the guided mode and wavelength), and χ is a quantity related to thespecific mode that is guided. For a TE₁₁ mode, χ is equal to 1.841.Values of χ corresponding to other guided modes will be readily apparentto the ordinarily skilled practitioner.

It should be noted in this regard that the transformation of thefundamental dielectric mode to the TE₁₁ metallic mode may be incomplete.Some finite portion of the energy of the dielectric mode may be coupledinto metallic modes other than the TE₁₁ metallic mode. However, the TE₁₁mode generally experience less attenuation, in the evanescent region,than any of the other metallic modes. For that reason, a light wave thathas traversed the evanescent region substantially contains only the TE₁₁metallic mode. (It should further be noted that because of the finiteconductivity of actual metal coatings, a perturbed TE₁₁ mode isexpected; i.e., a mode in which the electric field has a smalllongitudinal component.)

Because substantial attenuation takes place in the evanescent region, itis desirable to make that region as short as possible. However, otherfactors, to be discussed below, many militate for differing evanescentlengths in differing applications.

It is desirable for the cladding outer surface in the tapered portion ofthe fiber to be substantially smooth in order to reduce scattering oflight from the HE₁₁ mode near the cladding outer surface, and in orderto receive a metal coating that is relatively thin (preferably, lessthan about 1500 Å thick) and also substantially free of defects capableof leaking optical radiation. A cladding surface is here considered tobe substantially smooth if no surface texture is apparent on a scale ofmore than about 50 Å as observed in a scanning electron micrograph(SEM). Surfaces of such desirable smoothness are readily produced byheating and drawing the fiber.

It is desirable for the end flat to be substantially planar, and to beoriented substantially perpendicular to the axial direction of thefiber. The end flat is here considered to be substantially planar if,over the surface of the end flat, examination by SEM reveals nodeviation from planarity greater than about 100 Å by any feature of anylateral extent.

It is desirable for the edges of the end flat to be relatively sharplydefined. The edges are here considered to be sharply defined iexamination by SEM reveals an average radius of curvature, at the edge,of less than about 100 Å. End flats of such desirable planarity, andhaving edges of such desirable sharpness, are also readily produced byheating and drawing the fiber.

One method for metal-coating the terminal portion of the fiber(preferably including at least the transition region, as noted above)involves the use of an evaporation source of, e.g., aluminum. As shownin FIG. 6, the fiber 230 is not oriented end-on toward the source 290during evaporation and deposition of the aluminum. Instead, the probeend of the fiber is pointed away from the source, such that the end flatlies in a shallow relative to the direction of incidence of thebombarding metal. Typically, the fiber axis is inclined at an angle θ ofabout 75° relative to a line drawn from the source to the fiber tip.During deposition, the fiber is rotated about its own axis in order touniformly coat all sides of the fiber. When such a method is employed, acoating is readily produced that smoothly covers the cladding outersurface, but leaves the end flat substantially bare of any metal. Withreference to FIG. 7, the above-described method is useful for producinga probe in which the optical aperture 300 corresponds to the entiresurface of the end flat. Because the spatial resolution of the probe isdetermined mainly by the aperture diameter, it is desirable, forhigh-resolution applications, to make the end flat diameter quite small.Thus, the end flat diameter in such a probe would generally be madesmaller than the cutoff diameter. For example, the cutoff diameter for5145 Å light from an argon ion laser is typically about 2000 Å in afiber of the exemplary dimensions discussed above. A typical,corresponding end flat diameter is about 500-1000 Å, and end flats assmall as about 200 Å, or even smaller, are readily made by heating anddrawing the fiber. In general, the average thickness of the aluminumlayer is preferably not less than about 750 Å, because substantiallythinner layers may be subject to excessive optical leakage. (If theprobe is used for both illuminating the object and collecting light fromthe object, an even smaller thickness may be acceptable.) The thicknessis preferably not greater than about 1500 Å, because it is desirable tominimize the total diameter of the probe tip and to make the coating assmooth as possible. The total diameter is the sum of the end flatdiameter and the metal thickness bounding the end flat at diametricallyopposite positions. It is desirable to minimize this total diameter inorder to provide a probe tip that can be inserted within relativelynarrow cavities or crevices in the sample surface, and that can, moregenerally, penetrate as deeply as possible into the near field of thesample.

Significantly, a probe of the above-described type has an evanescentregion extending from the cutoff diameter to the end flat. Such anevanescent region is relatively long, by comparison to an alternatedesign to be described below. For example, with a cutoff diameter of2000 Å, an end flat diameter of 500 Å, and a taper angle of 15°, theevanescent length is 5600 Å. In such a distance, a significant amount ofattenuation may take place. Consideration of the attenuation militatesfor making the taper angle as large as is feasible in order to shortenthe evanescent length. However, it must also be considered that anarrower taper angle makes it easier to penetrate crevices and toapproach step-like surface features more closely.

According to a preferred method for tapering the fiber, the fiber isfirst mounted in a commercial pipette puller. The fiber is heated priorto and during drawing. An exemplary heat source is a carbon dioxidelaser. The fiber is drawn until it breaks. Controllable parametersinclude the incident illumination intensity (which determines theheating rate), the pulling force applied to the fiber ends, and thenumber of individual pulling steps that are applied. Generally, thefiber is finally broken by a "hard pull," that is, a pulling stepconducted with sharply increased force. The velocity of the fiber endsat the moment of application of the hard pull is also controllable, asis the time delay between cessation of heating and application of thehard pull.

We have discovered that a given set of process parameters producesprobes having highly reproducible characteristics. The taper angle isreadily increased by any selection or combination of reducing theheating rate, reducing the pulling force, pulling in multiple steps, orreducing the extend of the heated region. The end flat diameter isreadily increased by increasing the heating rate, or increasing thepulling force, or both. In addition, the use of a fiber having a highertemperature glass composition provides a bigger taper angle and asmaller tip. Desirable combinations of process parameters will bereadily apparent to the ordinarily skilled practitioner after minimalexperimentation.

An alternative type of probe is illustrated in FIG. 8. In this type ofprobe, the metal coating is deposited over the end flat as well as onthe cladding outer surface. As discussed above, the metal thickness onthe cladding outer surface is desirably about 750-1500 Å thick. Themetal layer overlying the end flat is desirably about 250-500 Å thickbecause it needs to be thick enough to substantially eliminate straylight, but also as thin as practicable in order to minimize theevanescent length. The optical aperture 300 in this case corresponds,not to the entire end flat, but only to a portion of the end flat. Themetal layer overlying the end flat is formed into an annulus having anopening which defines the optical aperture. Significantly, the openingmay be centrally located on the end flat, or, alternatively, it may bedisplaced from the center of the flat. A non-central aperture may bedesirable where multiple modes are expected to be present in the tipregion. Because not all modes will couple efficiently to a centrallylocated aperture, a non-central aperture in such a case may provide moreefficient output coupling. In particular, a non-central aperture isexpected to give the optimal output coupling for the TE₁₁ mode.

The central opening is formed, for example, by initially providing afiber that incorporates an embedded rod of glass that is more rapidlyetched than the host glass of the fiber. (An example is borosilicateglass embedded in fused silica.) The end of the fiber is exposed to achemical etchant after pulling but before the fiber is metallized. Theetchant produces a cavity in the end flat. The cavity is shadowed duringmetallization of the end flat, resulting in an opening in the metallayer.

A typical aperture made by this method is about 200-2000 Å in diameter,and the same method is readily employed to form apertures even larger orsmaller.

The metal coating is exemplarily deposited from an evaporation source,as discussed above in connection with the bare-tipped probe. However,two separate deposition steps are employed: one for coating the claddingouter surface, as described above, and a second step at a differentfiber orientation for coating the end flat.

Because in this type of probe, the aperture is not determined by the endflat diameter, it is not generally necessary for the end flat diameterto be smaller than the cutoff diameter, even for high-resolutionapplications. Accordingly, in order to minimize attenuation, it isdesirable for the end flat diameter to be, approximately, at least thecutoff diameter. However, as discussed above, it is desirable tominimize the total probe tip diameter in order to be able to penetratecrevices. For this reason, the end flat diameter is preferably also notsubstantially greater than the cutoff diameter.

It should be noted in this regard that the aperture diameter isgenerally smaller than the cutoff diameter, and therefore the centralopening in the metal layer overlying the end flat defines a metallicwaveguide in which the optical field is evanescent. However, because thecoating is typically only about 500 Å thick or less, the evanescentlength is much smaller than it is in the case of the bare-tipped probediscussed previously. Clearly, in order to minimize attenuation, it isdesirable to make the metal coating on the end flat as thin as ispracticable. An acceptable range for metal thicknesses on the end flatis about 250-500 Å, and, as noted, 500 Å is typical.

We have calculated the ideal power attenuation expected in theevanescent portion of a bare-tipped probe. The corresponding powertransmission coefficient is conveniently expressed in terms of χ, which,as noted above, is a characteristics of the guided mode, the taper angleβ, and a dimensionless parameter α, which is defined by

    α=χ/nk.sub.0 a,

where a is the aperture diameter, and as noted above, k₀ is thefree-space wavenumber of the guided light, and n is the relevant valueof the waveguide refractive index. It is helpful to note that α is equalto the ratio of the cutoff diameter to the aperture diameter. Whenexpressed in decibels, the power transmission coefficient correspondingto ideal evanescent attenuation is given by: ##EQU1## We have succeededin making probes, having various values of α ranging between 1 and 10,and various values of β ranging between 5° and 20°, in which the overallmeasured power transmission coefficient falls below T_(ev) by no morethan 10 dB.

At least for values of α ranging between 2 and 8, T_(ev) may be roughlyapproximated by -2χα/tan β dB. Accordingly, we have succeeded in makingprobes for which the overall measured power transmission coefficient,denoted T, satisfies

    T(db)>-(2χα/tan β)-10.

The inventive optical system is useful, inter alia, as a manufacturingtool. For example, the probe is readily used to conduct actinicradiation from a light source to a small region of a workpiece surfacesituated adjacent the optical aperture of the probe. Thus, for example,a workpiece surface may be coated with a photosensitive layer such as aphotoresist, and a pattern formed in the layer by moving the proberelative to the layer while transmitting light through the probe suchthat light exits the aperture and falls on the layer, thereby exposingthe layer. Additional steps are then readily performed, leading, forexample, to the completion of an article of manufacture. Exemplarily,the article of manufacture is a semiconductor integrated circuit, andthe additional steps include developing the photoresist and subjectingthe resist-coated surface to an etchant such that a layer underlying thephotoresist is patterned. Significantly, not only semiconductor wafers,but also glass plates intended for use as photolithographic masks, arereadily patterned in this fashion.

In an alternative process, actinic radiation from the probe is readilydirected to a workpiece surface in order to cause deposition of materialon the surface. For example, the surface may be exposed to anorganometallic vapor or solution. Actinic radiation impinging on thesurface may excite local decomposition of one or more constituents ofthe vapor or solution, producing, e.g., a metallic residue that adheresto the surface.

The inventive optical system is also useful as an inspection device on amanufacturing line. For example, with reference to FIG. 9, in themanufacture of semiconductor integrated circuits, a semiconductor waferis often provided (Step A), having a surface that is patterned at one ormore stages of the manufacturing process. The patterns that are formedhave characteristic dimensions, sometimes referred to as "line widths,"that must generally be kept within close tolerances. The inventiveoptical system is readily used for measuring (Step D) line widths, suchas the width of metallic conductors on a wafer, or the length of gatesformed in metal-oxide-semiconductor (MOS) structures on a wafer. Suchline widths can then be compared (Step E) with desired values. Processparameters, exemplarily lithographic exposure times or etching times,which have been initially set (Step B) are readily adjusted (Step H) tobring the measured dimensions within the desired tolerances. Additionalsteps (Step G) toward completion of a manufactured article ar thenperformed. The measuring step includes situating the probe adjacent thepatterned surface (Step I) and impinging (Step J) light upon, anddetecting (Step K) light from the surface such that the light source anddetector are optically coupled via the inventive probe, which issubstantially as described above.

The inventive optical system is also useful for inspecting bit patternsin magnetic digital storage media such as magnetic disks. Becausemagnetic storage media generally exhibit a Faraday rotation of polarizedlight, the bit patterns, which are characterized by modulatedmagnetization directions, are readily visualized by inspection using,e.g., crossed polarizers. Thus for example, the inventive optical systemmay be used for reflection-mode imaging of the medium, using a polarizedlight source and including a polarizing filter in front of the detector.In a manufacturing process that involves impressing such media with bitpatterns having predetermined properties, relevant process parametersmay be adjusted to bring the detected bit patterns into conformity withthe desired patterns.

In at least some cases, the laser light source is itself a source ofpolarized light. In other cases, it may be desirable to pass light fromthe source through a linearly polarizing film. Before such light iscoupled into optical fiber 70 (see FIG. 1), it is often useful to passit through a half-wave plate and then a quarter-wave plate. Theorientations of these plates are readily adjusted in order to compensatefor birefringence in the optical fiber. (A polarization preserving fibermay be useful, but is not necessary). The linearly polarized componentof light emerging from the fiber is optimized by, e.g., visuallydetecting it through a second linearly polarizing film while adjustingthe half-wave and quarter-wave plates.

Other applications of the inventive optical system for manufacturing andinspection will be readily apparent to the skilled practitioner.

For example, the inventive optical system is also useful for impressingdigital data for storage on an optical or magnetic storage medium, andfor reading such stored data from the storage medium. For example, it iswell known that data can be recorded on magnetizable metallic films inthe form of patterns of spots having local magnetization that differs indirection from the magnetization of surrounding film portions. This datastorage technology is described, for example, in R. J. Gambino, "OpticalStorage Disk Technology," MRS Bulletin 15 (Apr. 1990) pp. 20-24, and inF. J. A. M. Greidanus and W. B. Zeper, "Magneto-Optical StorageMaterials," MRS Bulletin 15 (Apr. 1990) pp. 31-39.

A typical magnetic storage medium is a layer which comprises anamorphous alloy of one or more of the rare-earths and one or more of thetransition metals. (Alternative magnetic storage materials includecobalt-platinum or cobalt-palladium multilayer films, and magnetic oxidematerials such as ferrites and garnets.) A spot representing, e.g., abit of digital data, is written by exposing the medium to a magneticfield and optically heating the spot area above the Curie temperature orabove the compensation point of the medium. (In some cases, the internaldemagnetizing field of the medium is sufficient to cause local reversalof magnetization, and an external field need not be applied.) Such spotsare conventionally made about 1 μm in diameter. However, the inventiveoptical system is readily employed to make smaller spots, exemplarilyspots having diameters of about 0.2-0.5 μm, and even as small as 0.06 μmor less. Such smaller spots can also be read by the inventive opticalsystem.

In a currently preferred embodiment, the magnetic storage medium is athin, amorphous film of an alloy comprising at least one rare earth andat least one transition metal. An exemplary such alloy is terbium-ion.The aperture of the near-field probe is situated less than about oneillumination wavelength from the surface of the medium. (If larger spotsare desired, the probe is readily positioned more than one wavelengthfrom the medium.) The illumination wavelength is selected to giveadequate heating to the medium. For example, a terbium-iron film isreadily heated by a dye laser which is pumped by a YAG laser and emitsroughly nanosecond pulses at a wavelength of roughly 600 nm. Light fromthe laser is directed into the inventive probe via an optical fiber, andis impinged from the probe tip onto the recording medium. A typical,local temperature change required to write a spot is about 150° C.Although writing can be achieved using either a continuous or pulsedlaser for illumination, a pulsed laser is preferable in order to relaxthe average power requirements of the laser while heating a relativelysmall spot.

Significantly, the invention is intended to encompass not only magneticfilm recording media, but also other media that are capable of beingwritten by an illumination source. Such media include, for example,polycrystalline films (of, e.g., tellurium-doped indium antimonide) thatare locally heated to, e.g., a temperature which exceeds the meltingpoint of the film by a laser pulse and rapidly quenched to an amorphousstate. The cooling rate can be influenced by, e.g., appropriatelyshaping the time dependence of the laser pulse.

It should be noted in this regard that direct optical heating is onlyone of several possible mechanisms for the local heating that results inthe recording of data in magnetic or other storage media. For example,indirect optical heating may also be useful for this purpose. Byindirect heating is meant heating of the entire probe tip by opticalabsorption in, e.g., the metal cladding. Heat is then transferredthrough a projection on the probe tip to a localized spot in the storagemedium. Other examples of potentially useful heating mechanisms areohmic heating, in which an electric current is passed through the probetip, and field-emission heating, in which a difference in electricalpotential is created between the probe tip and the surface of thestorage medium.

Spots representing recorded data are typically written in a track in therecording medium. Such a track extends, e.g., circumferentially on arotating disk. The diameters of spots written in accordance with theinstant invention are typically much smaller than the width of such atrack. Accordingly, multiple such spots are advantageously written inbands extending substantially transversely across the track. Oneadvantage of the invention is that the spots in such bands can be readsimultaneously by a linear array of inventive near-field probes.

In a currently preferred method of reading a pattern impressed on amagnetic film recording medium, linearly polarized light emitted by theinventive probe is transmitted through the medium, and conventionalmeans are used to collect a portion of such transmitted light and topolarization analyze it. For reading, preferred wavelengths are thosehaving a maximal optical response (i.e., maximal rotation of thedirection of polarization of the light as it traverses the magneticmedium). For transition-metal-rare-earth media, such wavelengths willtypically lie in the near-infrared or visible spectrum.

In an alternate embodiment, the probe is used both to impinge polarizedlight onto the surface of the medium, and to collect a portion of suchlight that is reflected from the surface. As a result of eithertransmission through, or reflection from, a magnetized spot in themedium, a polarization rotation of, typically, about 0.5° is produced.The effect of such rotation is to modulate the intensity of lighttransmitted through an analyzer and subsequently detected. Suchmodulation is readily decoded to reproduce the information recorded inthe medium. As is well known, such information may, for example, berecorded sound, images, text, or digital data.

In alternate embodiments, such as those involving phase changes,modulation is typically provided by changes in reflectivity, rather thanby polarization rotation. Such changes are also readily detectable usingthe inventive probe to collect reflected light.

The inventive microscope is also advantageously employed for imagingapplications in biological research and clinical medicine. Inparticular, the inventive microscope overcomes, in at least some cases,the problem of low signal level that often detracts from the usefulnessof prior-art NSOM systems for medical and biological imaging. Thus, forexample, the inventive microscope is readily used to image sectionedsamples of biological tissue in order to find and identify physicalpathologies in the tissue. In a similar manner, the inventive microscopeis readily used to examine the distribution in sectioned tissue samplesof materials that are detectable by their inherent appearance orfluorescence, as well as materials that are labeled by, e.g.,fluorescent dyes.

The inventive microscope is also useful, in genetic clinical andresearch applications for imaging chromosomes while they are in, e.g.,the metaphase state. In particular, chromosomes or portions ofchromosomes that are labeled with fluorescent material are readilyidentified using the inventive microscope. Methods of fluorescentlabeling, which generally involve reacting cell nuclear material withmaterial that includes fluorophores, are well-known in the art and neednot be described in detail herein.

Fluorescent imaging is readily achieved using the microscope in eitheran illumination mode (FIG. 1) or a collection mode (FIG. 2). In theformer, electromagnetic radiation capable of inducing the sample to emitflourescent light impinges on the sample from the probe and theresulting fluorescence is collected by, e.g., a conventional microscopeobjective. In the latter, the exciting radiation is conventionallyimpinged on the sample, and the fluorescent light is collected by theprobe.

As noted, a preferred probe 20 (see FIG. 1) is made by tapering asingle-mode optical fiber. It should be noted that in at least somecases a useful probe can also be made by tapering and coating a multiplemode fiber. Depending on the dimensions of a multiple mode fiber(relative to the guided wavelength), a greater or lesser number of modeswill be guided. In general, it is expected that the fewer the number ofguided modes (i.e., the more closely the fiber resembles a single-modefiber), the greater will be the signal-to-noise ratio achieved for agiven aperture.

Also as noted, an exemplary opaque coating for the probe is a metal suchas aluminum. More generally, an appropriate coating is one consisting ofa material in which the guided radiation has a low penetration (skin)depth. Aluminum is a preferred material when the radiation lies in,e.g., the visible spectrum, but semiconductors such as silicon may bepreferable where, e.g., ultraviolet radiation is to be guided.

A tapered optical fiber is only one example of a broader class ofoptical waveguides that can be used as probes in accordance with theinvention. The general characteristic of such probes is that they have aportion distal the probe tip which supports at least one propagating,dielectric mode of electromagnetic radiation, and a portion proximal theprobe tip at least part of which supports evanescent or propagatingmetallic modes. Such a probe has a core region and at least one claddingregion. The dielectric mode or modes are determined by the boundaryconditions at the interface between the core and the cladding. Bycontrast, the metallic mode or modes are defined by the boundaryconditions at least at one metal layer (or, more generally, an opaquelayer having a penetration depth much smaller than the wavelength of theguided radiation) which is situated adjacent the cladding, distal thecore region.

At least a portion of the probe is tapered at an angle β, and thetapered portion includes both dielectric and metallic waveguide regions.At least partly as a result of the taper, an electromagnetic wavepropagating in the probe is transformed between dielectric and metallicmodes as it passes between the respective regions. The angle βis"adiabatic" in the sense that the dielectric region of the waveguideguides light of the low-order dielectric modes to the metallic regionwith high efficiency. If, by contrast, the taper were too sharp (i.e., βtoo large), a substantial fraction of the radiation injected into thesemodes in the dielectric region would be lost to retroreflection andscatter.

It is preferable for the dielectric region to support relatively few,e.g., less than 10, propagating modes, and more preferably, only onesuch mode. It is an advantageous property of dielectric waveguides thatalthough the injected radiation can be efficiently coupled into thecladding, which may present a relatively large cross section to theradiation source, the injected radiation can be limited to only one or afew propagating modes that are bound to the core. By contrast, more than10, and even as many as one million or more, propagating modes willtypically be excited when radiation is injected into a hollow metalwaveguide, or a metal waveguide filled with a single, dielectricmaterial, which has a diameter of, e.g., 100 μm. When such a waveguideis tapered, a substantial fraction of the propagating radiation islikely to couple into radiative modes which are retroreflected,absorbed, or scattered out of the waveguide. Consequently, this fractionfails to propagate to the metallic portion, where the surviving light isconfined to subwavelength dimensions.

Each propagating dielectric mode has a typical electric field profile.The amplitude of this profile, at the cladding outer surface, isinitially insignificant relative to the amplitude at the core, even if ametal layer or coating is absent. This condition assures thatdielectric, and not metallic, modes are present. In the absence of ametal coating or layer, the amplitude at the cladding outer surfaceincreases within the taper region as the width or diameter of thewaveguide decreases. As a consequence, there is some part of the taperregion where the boundary conditions imposed by the metal layer orcoating become comparable in importance to the dielectric boundaryconditions. This condition defines a transition region. At one end ofthe transition region region, the mode is predominantly dielectric incharacter, and at the other end it is predominantly metallic incharacter. We have observed efficient coupling of energy at leastbetween the lowest propagating dielectric mode and the least evanescentmetallic mode, which we currently attribute to a large overlap integralbetween these modes.

It is important for the waveguide to be hybrid in character, with adielectric portion as well as a metallic portion. The dielectric portionmakes it possible to efficiently couple radiation from a conventionallight source into a few well-confined dielectric modes in the probe,and/or to couple radiation from a few such modes into a conventionallight detector. The dielectric portion also provides substantiallylossless transmission of radiation through the tapered region during,e.g., illumination-mode operation. Near the tip, by contrast, thewaveguide should be metallic in character, because a metallic waveguideprovides much better confinement of the electromagnetic field. Whereasthe optimum confinement in a purely dielectric waveguide (as defined bythe full width at half-maximum of the components of the electromagneticfield) is about (λ/2) (i.e., one-half the guided wavelength), a metallicwaveguide can provide confinement as good as (λ/10) or even better.Because light is efficiently coupled into the waveguide and efficientlytransferred between the dielectric and metallic portions, relativelylarge signal-to-noise ratios can be achieved. This, in turn, makes itfeasible to use relatively small apertures, thereby achieving relativelyhigh image resolution. Moreover, coupling into a few well-confined modestends to suppress radiative losses through pinholes in the opaque layeror coating, which can otherwise result in an undesirably high backgroundradiation level. This enhances the reliability of processes formanufacturing probes.

One example of a waveguide useful in this regard is modified opticalfiber probe in which a conductive wire is situated on the longitudinalaxis in the terminal metallic portion, such that a coaxial waveguidestructure is formed. This structure is capable of supporting at leastone propagating mode.

One example of a waveguide structure useful in this regard, alternativeto an optical fiber waveguide, is a thin-film waveguide disposed on aplanar substrate. We believe that useful embodiments of the inventiveprobe can be based on this as well as other alternative waveguidestructures, provided that such structures include the generalcharacteristic described above.

Probes based on optical fibers, thin-film waveguides, or otherwaveguiding structures will typically terminate in an end flat, with anoptical aperture defined in the end flat. However, in some cases it maybe desirable to admit or emit light through an aperture formed at ornear the edge of the terminal end flat. Such an aperture is readilyformed adjacent to the end flat by, e.g., overcoating the end flat withopaque material, and then removing opaque material down to anappropriate plane. Such a plane will be inclined relative to thewaveguide longitudinal axis, and will intersect the end flat at or nearan edge thereof.

A thin-film waveguide disposed on a planar substrate has severaladvantageous properties. For example, a probe based on this kind ofwaveguide can be mass produced in large groups on single substrates suchas silicon wafers using, e.g. lithographic processing, resulting inreduced unit cost. Moreover, probes of this kind can be manufactured inlarge arrays for use, e.g., in applications which require parallelreading or writing operations.

Such a waveguide would comprise vitreous or non-vitreous material, andwould include a core layer, a cladding layer underlying the core layer,and, optionally, a cladding layer overlying the core layer. (In someembodiments, a portion of the upper surface of the core layer may beeffectively in direct contact with air or the ambient atmosphere orvacuum, which, by virtue of the refractive index difference across thecontracting surface, would behave like an upper cladding.)

An example of a so-called "channel waveguide" 400, formed, e.g., on aplanar surface of a silicon substrate 410, is illustrated in FIG. 10.Well-known methods of material deposition, such as chemical vapordeposition or sputter deposition, are used to form layers 420-460. Thelateral extent of one or more layers is exemplarily defined bylithographic patterning and etching. The deposited layers include loweropaque layer 420 and upper opaque layer 460, which are composed of amaterial having a relatively short penetration depth for theelectromagnetic radiation that is to be guided. (By a "short"penetration depth is meant a depth which is much smaller than therelevant wavelength λ, exemplarily, (λ/10) or less.) The opaque layersare analogous to layer 280 of FIG. 5. The deposited layers furtherinclude lower cladding layer 430, optional upper cladding layer 450, andcore layer 440. Exemplary compositions for the respective layers are:for opaque layers, aluminum (desirable for relatively high opacity) orchromium (desirable for relatively high melting point); and for the coreand cladding layers, silica-based glass.

In a planar waveguide of the kind described herein, the TE₀₁ mode willgenerally be the sole propagating metallic mode if the width W of thewaveguide is less than (λ/2n), where λ is the guided wavelength and n isthe refractive index of the cladding. (By the width W is meant thethickness of the combined core and cladding layers.) This mode will bethe only mode of practical significance in the metallic portion of thewaveguide. This mode will be strongly linearly polarized, with theelectric field vector oriented perpendicular to the metal coating. Theelectromagnetic field components of this mode are described, e.g., in J.B. Marion and M. A. Heald, Classical Electromagnetic Radiation, 2d. Ed.,Academic Press, New York (1980) p. 191.

For a sufficiently small core in the dielectric region, there willgenerally be two propagating modes, namely the TE₀ and the TM₀ modes.The electromagnetic field components of these modes are described. e.g.,in A. W. Snyder and J. D. Love, Optical Waveguide Theory, Chapman andHall, London (1983), p. 242.

At least a portion of core layer 440 is adiabatically tapered at angleβ. The taper is exemplarily formed by varying, according to longitudinalposition, the exposure time of the substrate to the source of depositingcore material and the source of depositing cladding material.(Analogously to the tapered fiber probe, it is preferable to maintain anapproximately constant ratio of core thickness to cladding thickness inthe taper region.) The waveguide terminates in an end flat 480, whichmay be defined by the edge of the substrate during the depositionprocess or, alternatively, by cutting and polishing the substrate afterdeposition.

The optical aperture is defined in the end face by the spacing betweenthe opaque layers. As long as the transverse length L of the aperture isgreater than (α/2n), where λ is the guided wavelength and n is therefractive index of the cladding, the terminal portion of the waveguidewill support at least one propagating metallic mode, regardless of thewidth W of the waveguide. As a consequence, the waveguide canadditionally be tapered in the transverse direction, as long as therelation L>(λ/2n) is satisfied. This additional taper, depicted in FIG.11, is exemplarily formed by lithographically patterning and etching thewaveguide.

As noted, the metallic waveguide portion will typically have a singlepropagating mode that is linearly polarized. This polarization effect isvery useful for magneto-optical imaging, in which image contrast isproduced as a result of the rotation of the plane of polarization oflight in an appropriate medium by magnetic fields.

Appropriate optical elements are readily provided for couplingelectromagnetic radiation into or out of probe 400 from or to, e.g., anoptical fiber. However, in preferred embodiments, an optical fiber isnot used. Instead, a diode laser is coupled to rear face 500 eitherdirectly, or through a planar waveguide. It is expected that modecoupling from probe 400 to a diode laser will be more efficient thanmode coupling to an optical fiber because like the probe, a diode laserhas a linear geometry, whereas an optical fiber has a circular geometry.

A probe such as probe 400 will be especially useful for applicationswhich require miniaturization of components. Those applications mightinclude, for example, consumer electronics which read data or audio orvideo information from a dense optical or magneto-optical storagemedium.

EXAMPLE

A 3-μm, single-mode fiber (FS-VS-2211) having a 450 nm cutoff, was drawnin a Mod. P-87 micropipette puller manufactured by Sutter Instruments,while heating the fiber with a 25-watt, 3-mm spot from a 25-watt carbondioxide laser. The micropipette puller was programmed to provide a hardpull at a setting of 75 (range 0-255), a "velocity at pull" at a settingof 4 (range 0-255), and a time delay of 1 (range 0-255). The tipproperties that were obtained were: a 12° taper angle, a 670-Å-diameterend flat, and a value for α of about 3. One fiber end was placed in arotator and evaporation coated with about 1260 Å of aluminum at a basepressure of about 10⁻⁶ torr. The tapered end was then mounted in apiezoelectric tube in the optical arrangement of FIG. 1. One milliwattof 514.5-nm light from an argon ion laser was coupled into the fiber.The optical power output at the fiber tip was measured to be about 1.1nanowatt, corresponding to an overall power transmission coefficient ofabout -60 dB. When used to form an image of a sample surface, the probeprovided a spatial resolution of about 25 nm.

We claim:
 1. A method for examining a biological specimen, comprisingthe steps of: impinging incident light on the specimen; detecting lightemitted from a first surface of the specimen at least at one emissionwavelength; constructing a magnified image of a portion of the specimenfrom the detected light; and visually examining the magnified image;wherein the detecting step comprises the steps of:a) admitting a portionof the emitted light into a waveguiding body through an aperture that isdistant from the first surface by no more than the emission wavelength,and that is smaller, in at least one lateral dimension, than theemission wavelength; b) exciting a guided metallic mode ofelectromagnetic radiation within the waveguiding body; c) adiabaticallyconverting a portion of the light within the waveguiding body from themetallic mode to at least one guided dielectric mode; and d) receivingthe adiabatically converted light at a photodetector.
 2. The method ofclaim 1, wherein the specimen has a second surface opposite the firstsurface, the impinging step comprises impinging light on the secondsurface, and the light emitted by the first surface is light that hasbeen transmitted through the specimen.
 3. A method for examining abiological specimen, comprising the steps of: impinging incident lighton the specimen; detecting light emitted from a first surface of thespecimen at least at one emission wavelength; constructing a magnifiedimage of a portion of the specimen from the detected light; and visuallyexamining the magnified image; wherein the impinging step comprises thesteps of:a) admitting incident light having at least one incidentwavelength into an optical waveguiding body such that a guideddielectric mode is excited within the waveguiding body; b) adiabaticallyconverting a portion of the incident light from the guided dielectricmode to a guided metallic mode; and c) emitting a portion of theadiabatically converted light from an aperture that is distant from thefirst surface by no more than the incident wavelength, and that issmaller, in at least one lateral dimension, than the incidentwavelength.
 4. The method of claim 1 or 3, wherein the impinging stepcomprises impinging light on the first surface, and the detecting stepcomprises detecting reflected light.
 5. The method of claim 1 or 3,wherein: the impinging step comprises impinging, on the first surface,light capable of exciting fluorescence in the specimen; and thedetecting step comprises detecting fluorescent light.